<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    #demo {
      color: blueviolet;
    }
  </style>
</head>

<body>
  <!-- ID具有唯一性 -->
  <ul>
    <li>
      <p id="demo">我是p1标签</p>
    </li>
    <li>
      <p>我是p2标签</p>
    </li>
    <li>
      <p>我是p3标签</p>
    </li>
  </ul>
</body>

</html>